A friction stir-welding method (hereinafter referred to as FSW) has been known wherein a metal rod (hereinafter referred to as a welding tool) made of a material having a hardness higher than that of a member to be welded is inserted into a portion of the member to be welded, followed by rotating the welding tool while moving the tool, thereby to carry out welding by a friction heat generated between the welding tool and the member to be welded. This method is disclosed in Patent document 1, for example. FSW is a method by which the member to be welded is softened by friction heat between the welding tool and the member, wherein rotation of the welding tool results in plastic flow phenomenon of the member, whose principle is different from arc welding where a member to be welded is melted.
A spot welding method employing the principle of FSW has been known in patent documents 2 and 3, for example.
Patent document 1: Japanese patent No. 2,712,838 (WO 93/10935)
Patent document 2: Japanese patent No. 3,400,409
Patent document 3: Japanese patent No. 3,429,475